Wearable exoskeletons have been designed for medical, commercial, and military applications. Medical exoskeleton devices restore and rehabilitate proper muscle function for patients with disorders affecting muscle control. Medical exoskeleton devices have systems of motorized braces that can apply forces to a wearer's appendages. In a rehabilitation setting, medical exoskeletons are typically controlled by a physical therapist who uses one of a plurality of possible input means to command an exoskeleton control system. In turn, the exoskeleton control system actuates the position of the motorized braces, resulting in the application of force to, and typically movement of, the body of the wearer. Medical exoskeletons can also be used outside of a therapeutic setting to grant improved mobility to a disabled individual. Commercial and military exoskeletons are used to alleviate loads supported by workers or soldiers during their labor or other activities, thereby preventing injuries and increasing the stamina and strength of these workers or soldiers. Tool-holding exoskeletons are outfitted with tool-holding arms that support the weight of a tool, reducing user fatigue by providing tool-holding assistance. Each tool-holding arm transfers the vertical force required to hold the tool through the legs of the exoskeleton rather than through the wearer's arms and body. Similarly, weight-bearing exoskeletons transfer the weight of an exoskeleton load through the legs of the exoskeleton rather than through the wearer's legs. In some cases, weight-bearing exoskeletons are designed to carry a specific load, such as a heavy backpack. In other cases, military weight-bearing exoskeletons support the weight of armor. Commercial and military exoskeletons can have actuated joints that augment the strength of a wearer, with these actuated joints being controlled by an exoskeleton control system, and the wearer using any of a plurality of possible input means to command the exoskeleton control system.
In powered exoskeletons, exoskeleton control systems prescribe and control trajectories in the joints of the exoskeleton, resulting in movement of the exoskeleton. These trajectories can be prescribed as position-based, force-based, or a combination of both methodologies, such as those seen in impedance controllers. Position-based control systems can be modified directly through modification of the prescribed positions. Similarly, force-based control systems can be modified directly through modification of the prescribed force profiles. Complicated exoskeleton movements, such as walking in an ambulatory medical exoskeleton, are commanded by an exoskeleton control system through the use of a series of exoskeleton trajectories, with increasingly complicated exoskeleton movements requiring an increasingly complicated series of exoskeleton trajectories. These series of trajectories can be cyclic, such as the exoskeleton taking a series of steps with each leg, or they can be discrete, such as an exoskeleton rising from a seated position into a standing position. In the case of an ambulatory exoskeleton, during a rehabilitation session and/or over the course of rehabilitation, it is highly beneficial for the physical therapist to have the ability to modify the prescribed positions and/or the prescribed force profiles depending on the particular physiology or rehabilitation stage of a patient. As exoskeleton wearers are each differently proportioned, variously adjusted or customized powered exoskeletons will fit each wearer somewhat differently, requiring that the exoskeleton control system take into account these differences in wearer proportion, exoskeleton configuration/customization, and exoskeleton-wearer fit, resulting in changes to prescribed exoskeleton trajectories.
While exoskeleton control systems assign trajectories to the joints of exoskeletons and control the positions of these joints, the actual forces applied to exoskeleton joints are exerted by actuators. These actuators can take many forms, as is known in the art, each with advantages and disadvantages in various applications. In current exoskeletons, the actuator exerting force on a joint typically includes an electric motor located proximate to that joint. Co-location of the actuator with the joint has advantages in terms of mechanical and design simplicity, but it has certain disadvantages. Foremost among these disadvantages is that adding a bulky electric motor to a joint increases the bulk of the joint, limiting maneuverability of the joint and exoskeleton in certain environments. In comparison, consider a human finger. The musculature exerting force on the joints of the finger is not located near the joints of the finger but rather in the forearm, with muscular contraction pulling on tendons that relay the force over distance to the joints. This has the advantage of minimizing the bulk of the fingers, allowing for both greater dexterity and closer placement of the fingers to each other. In addition, more muscle can be located in the arm than would fit on the fingers, allowing for greater strength. One mechanical actuation device, described in U.S. Pat. No. 4,843,921, uses a drive mechanism in which an electric motor twists a loop of cord, with this cord loop forming a helical structure and shortening as it is twisted, thereby causing the length of the cord loop to shorten and pulling the two ends of the cord loop closer together. In this way, the activation of the electric motor is used to apply a pulling force over distance through the cord loop. This allows for a design in which the motor driving movement of a joint is located at a position distal from the joint being moved.
It is conceivable that a similar motor-and-loop drive system could be used to power the larger joints of a human exoskeleton, such as the knee of a human exoskeleton. One major advantage of this design would be a compact exoskeleton knee, which would reduce knee bulk and weight, allowing for a more maneuverable exoskeleton in cramped environments, such as getting in and out of a vehicle. However, there are also a number of disadvantages to such a design. First, the mechanisms of most robotic/exoskeleton actuators allow the actuator to exert force in two joint movement directions. In the case of the knee, those movement directions are flexion and extension. Unfortunately, the motor-and-loop drive system is only able to cause a pulling motion, such that force is only applied to a joint in one direction. While some embodiments of this type of actuator could use springs or other similar devices to cause a joint to return to a position when the current to the motor is disengaged, this is not suitable for the forces required to move the large joints of a human exoskeleton and the body of a wearer. In addition, even high tensile strength cord, after being twisted and untwisted many times or having been subject to stress from a fall or misstep, may be at risk of breakage. Failure of the cord would result in uncontrolled joint movement and an unacceptable chance of injury to an exoskeleton wearer. In the case of medical exoskeletons, this injury concern is particularly significant, as wearers can be subject to conditions resulting in little or no control over the movement of the knee or other joints. Furthermore, regulatory bodies, such as the Food and Drug Administration, require that medical devices comply with safety guidelines. Unless the risks of such a design are addressed, it is unlikely to be approved for use by these agencies.
Based on the above, there exists a need in the art for an exoskeleton in which an actuator makes use of a motor to twist a loop of cord in order to cause movement of an exoskeleton joint at a distance from the motor, with the joint being subject to bidirectional movements, and with movement of the joint being impeded in the event of cord breakage, thereby preventing injury to a wearer of the exoskeleton or further damage to the exoskeleton. There also exists an unmet need for an exoskeleton in which a joint is subject to bidirectional movements through the action of a single motor. In addition, there exists an unmet need for a device that controls the position and separation of the strands of the cord loop twisted by the actuator, with this device acting in such a way that the strands of the cord loop are subject to reduced wear with each twist cycle, thereby prolonging the functional lifespan of the cord loop.